there are loads of ritek go4 around with
quality ratings from A++ to F, unless you
tell us the make you are using the answer
will have to be use better quality media
c,d,e,f rated media will surely fail over the 4 gig mark
its a classic sign of low quality media.
I alwasy use Datasafe GO4 riteks, and Ritek Plus 4x printable
(RICOHJPNR01) as these seem to be the best of the bunch.
(unless you are gonna splash out and buy Verbatim in jewel cases)